1. Field of the Invention
The invention relates to a vulcanizable rubber mixture for producing a leather-hard rubber containing a polymer base, sulfur, phenolformaldehyde reinforcement resin, accelerators and zinc oxide.
2. Description of the Prior Art
As is explained in German Utility Model No. 7 510 829, such a rubber mixture can be used for making a so-called composite profile which is made by simultaneous extrusion and vulcanization of two different grades of rubber. This composite profile has a region of leather-hard rubber which serves as the clamping part, and a second zone made integrally therewith, of ordinary soft, rubber-elastic rubber.
A vulcanizable rubber mixture for making such a leather-hard rubber in which 12 to 40 parts sulfur are used has been proposed. A disadvantage in the processing of such rubber mixture is that an exactly defined degree of heating from the machine side must be maintained accurately. Even the smallest deviations bring about an embrittlement and splintering of the leather-hard rubber and thereby, the clamping zones of the composite profile themselves, so that problems arise in punching and cutting the composite profile. In addition, sulfur dioxide is produced in the manufacture and processing of this mixture, creating a heavy disagreeable smoke.